1. Field of the Invention
This invention relates to an improved process for the removal of toxic cyanide and anionic metal species from alkaline solutions resulting from the mining and plating industries and to a process of flotation wherein recycled aqueous process stream water is employed to disperse a finely divided ore to provide a finely divided pulp, the recycled process stream water is an aqueous alkaline process stream from which toxic cyanide ion, anionic metal cyanide complexes or anions of arsenic or selenium have been removed.
2. Description of the Related Art
Gold occurs primarily as the native metal, alloyed with silver or other metals or as tellurides. It is commonly associated with the sulfides of iron, silver, arsenic, antimony and copper. Silver occurs as finely disseminated metal in rocks of hydrothermal origin as silver chloride, sulfide or tellurides and as complex sulfides with antimony and arsenic. Historical practice with ores containing native metal involve crushing, concentration of the gold or silver by gravity separation and recovery by amalgamation with mercury. Environmental concerns have resulted in abandonment of this process in most cases.
Two processes, heap leaching and carbon-in-pulp (CIP), are used for the recovery of gold from ore depending to a large extent on the grade and nature of the ore. Both processes result in waste streams consisting of dilute solutions containing low levels of cyanide, metal cyanide complexes, and depending on the ore, other toxic metal species such as arsenate or selenate. Low grade ores consisting of gold disseminated in largely siliceous rock are typically leached by piling the crushed ore on specially prepared pads to a depth of several feet and then continuously distributing an aqueous cyanide solution across the surface of the heap. As the cyanide solution trickles through the ore, the gold is leached from the ore as the soluble aurocyanide species. The gold bearing leach solution is collected at the bottom of the heap and pumped to a treatment facility for recovery of the gold. When the gold tenor in the leach solution drops to the point where it is no longer economically feasible to treat the ore, leaching is stopped and the heap is abandoned. At this point, the heap is saturated with dilute aqueous cyanide solution containing various other metal cyanide complexes as well as potentially other toxic metal species. This solution must be washed from the heap and treated to destroy the various cyanide species and remove the remaining toxic metal species. If the heap is not washed, these cyanide species and toxic metal species will continue to leach from the heap over time, resulting in an environmental threat to wildlife and groundwater.
There are many gold ore bodies in the world which contain gold as free gold metal together with gold associated with pyrite. The gold associated with pyrite is not amenable to recovery by direct cyanide leaching of the ore. The free gold, however, can be recovered by grinding the ore and leaching it with cyanide and recovering gold from the leach using activated carbon technology, or more recently ion exchange polymer technology for gold cyanide recovery as reported in "New Ion Exchange Polymers for Gold Cyanide Recovery"; Randol Forum, Perth '95. For those bodies containing pyrite a commonly used process is to recover the pyrite plus associated gold by flotation and to cyanide the free gold remaining in the ore. The pyrite is then roasted to expose the associated gold and the roast calcine is leached with cyanide to recover gold. Flotation is a process for concentrating the metal values as their sulfides from a sulfide ore into a concentrate that can be further treated by other processes such as smelting to recover the metals themselves. A general description of flotation can be found in the Encyclopedia of Chemical Technology (ECT) by Frank F. Aplan of The Pennsylvania State University, in Vol. 10, pp. 523-547.
In many parts of the world where flotation plants are located, availability of water for mineral processing is a major issue. In these arid regions, it is desirable to recycle the process water. During certain stages, cyanide may be added as a depressant as discussed in the ECT article. The cyanide, thiocyanate, and base metal cyanide complexes all report to the tailings dam water. Also the location on one metallurgical site of both cyanide leach operations and pyrite flotations means that the process waters used in pyrite flotation may become contaminated with free cyanide and associated thiocyanate and base metal cyanide anion complexes. These cyanide species are depressants for pyrite during flotation and it is desirable to remove the cyanide ion and the anionic cyanide metal complexes from the process water before the process water can be used in pyrite flotation. After treatment to remove the cyanide ion and anionic metal species cyanide complexes or anions of arsenic or selenium, the resultant purified water can then be returned to the flotation process.
Higher grade ores or ores in which the gold is locked in a matrix which is not easily leached are treated by CIP processes. The ore is finally ground and placed in a leaching vessel containing alkaline cyanide solution and carbon. The gold is adsorbed by the carbon as it is leached. The spent slurry is then subjected to a series of solid/liquid separation steps before being deposited in a tailings dam as a thickened slurry. Water continues to separate over time from these tailings. This water contains low levels of cyanide and metal cyanide species. It must be treated prior to discharge into the environment or before return to the leaching or flotation process. Alternatively, the thickened slurry can be treated by contacting with a solid phase adsorbent prior to discharge to the tailings bond to remove the cyanide and anionic metal species.
In both cases, the levels of cyanide species and toxic metals such as arsenic and selenium are very low, 1-10 ppm. In order to achieve essentially complete removal by standard oxidation processes with hydrogen peroxide or hypochlorite and precipitation processes, large excesses of reagents are required. If the toxic species can be concentrated, then treatment can be carried out much more efficiently with considerable savings in reagent costs.
Similarly, spent electroplating solutions and rinse solutions containing heavy metal cyanides such as zinc cyanide or cadmium cyanide are often dilute, requiring large excesses of reagents for their removal. Concentration of these species would also allow savings in reagent costs.